Thyristor tap changers of the stated category are usually also termed hybrid tap changers because they have, apart from the thyristors in the load diverter switch as electronic power switching means, also mechanical contacts, particularly mechanical selector contacts. It may be mentioned in passing that there are in addition also so-termed all-thyristor switches, such as, for example, known from WO 95/27931, which entirely dispense with movable mechanical switching elements, but are relatively large and complicated in construction, have not gained acceptance in practice and otherwise are also not the subject of the present invention.
The present invention is concerned with, rather, hybrid thyristor tap changers.
This category of thyristor tap changers can in turn be subdivided into two different apparatus types with different principles of function.
In the first instance there is known from DE 32 23 892 C2 a thyristor tap changer which operates according to the commutation principle. In that case the load switching over is carried out by a controlled commutation of the load current hence the name—from one antiparallel thyristor pair in one branch of the load diverter switch to the respective other thyristor pair in the other branch. Tap changers of this kind have been produced and used sporadically over 80 years as so-called leadthrough tap changers. In that case the active part of the load diverter switch is arranged on a leadthrough insulator post above the transformer tank in an air-filled housing, while the other part of the switch is immersed in the oil-filled transformer tank. The leadthrough insulator post is partly filled with insulating oil and connected with the ambient air by way of a silica gel seal. Disposed within the housing of the active part is a frame which receives the electronic subassemblies of the load diverter switch. The leadthrough support itself is fastened on an attachment flange; a carrier cage with terminal contacts is disposed in the load diverter switch oil chamber upwardly closed off by an attachment flange cover plate. Such a switch, however, has a very large space requirement, particularly due to the large porcelain leadthrough between the electronics housing above the actual transformer and the part, which is lowered into the transformer, of the apparatus with the carrier cage and the mechanical auxiliary switches. In addition, access to the individual components in the oil-filled region is also difficult, so that maintenance operations are complicated and awkward to perform. Overall, this type has not been able to gain acceptance in the past.
Further, as the other of the two types of apparatus a thyristor tap changer with transition resistance is known from WO 98/48432. In that case there is provided a single antiparallel thyristor pair with which a transition resistance lies in parallel. Not only the thyristor pair, but also the transition resistance can be actuated in a specific switching sequence and connected into the current circuit by specific mechanical switching-over contacts. In that case the load current briefly flows across the transition resistance during the load changeover and subsequently a circuit current, which is driven by the tap voltage of the regulating winding. The constructional build-up of a thyristor tap changer of practical execution based on this circuit is known from the company publication ‘OLTC Hybrid-Diverter Switch with Thyristors’ of the company ELIN OLTC GMBH, Austria, and from the article ‘Hybrid-Transformatorstufenschalter TADS-ein zukunftsweisendes Konzept zur Verlangerung der Wartungsintervalle’ in the periodical ‘e & i’, Vol. 11, 1999. The entire switch is in that case conceived as a complete insert able to lowered fully into the oil-filled transformer tank. It is disadvantageous in this construction that the thyristors are exposed to the hot transformer oil. This prejudices the long-term endurance of these electronic components which, as a rule, function reliably only in ambient temperatures up to approximately 100 degrees C. The problem is further aggravated by the fact that in the transition resistance—or, in practice, usually several transition resistances which are present—due to the current loading a quite substantial amount of energy has to be converted into additional heat, which puts at risk the function of the thyristors. In the case of the described known tap changers this has the consequence that only a limited number of load changeovers should be undertaken within a specific time period, so that the heat development caused by the transition resistances does not exceed a limit value. This is undesirable for numerous industrial cases of use. In this connection it has already been proposed to provide an additional temperature switch which blocks the motor drive of the tap changer, and thus temporarily stops the hybrid thyristor switch, when the environment of the thyristors exceeds a specific limit temperature which is not yet harmful. It has proved that this is similarly not practicable in numerous industrial cases of use; apart from that the problem is not solved by such a proposal, but merely a symptom cured.